Commercial plant growers artificially control the lengths of days and nights. this produces a response called
a. photoperiodism.
b. gravitropism.
c. thigmotropism.
d. phototropism.

The answer is A. photoperiodism.

Photoperiodism is an organism reaction to the lengths of days and nights. Some plants require longer days, and some require shorter days. By changing the lengths of days and nights, commercial plant growers are able to increase plants productivity.

Tropism is plant growth in a response to a stimulus.
As it name suggests, phototropism is plant growth in a response to light. 
Gravitropism is plant growth in a response to gravity.
Thigmotropism is plant growth in a response to contact

Humans can affect weathering and erosion in several ways.  humans cause  increases in acid rain and pollution, which increase the amount of  weathering  agents in the air and water, and then on land.for an easy answer: 1) acid rain2) pollution